Domeikite (Cu3As): reflected light properties, α- and β-modifications

α-domeikitis (low)

• Syngony: cubic
• Spatial Group: I4₃d
• Grid Parameter: a = 9.619; Z = 16
• Sustainability: below 225° (and below 100 MPa)
• Composition: close to the theory; Ag admixture

Microscopy: fine-grained, kidney-shaped and bunched aggregates; individual grains. The reflection is close to pyrite (~53%), the color is yellowish-cream; in aggregates with algodonite it is noticeably darker. The mineral is isotropic and relatively quickly covered with whiteness. VHN ~205.

β-domeikitis (high)

• Syngony: hexagonal
• Spatial Group: P3c1
• Parameters: a = 7.11; c = 7.27 (increasing with increasing Cu)
• Sustainability: ~225-827°
• Impurities: Co, Ni, Fe, Sb, S (<0.2%)

Microscopy: usually lamellar/spindle-shaped lamellae in α-domeikite (solid solution decomposition structures), or small grains. Reflection just below α-domeikite; hue slightly greenish-grayish; noticeable anisotropy. VHN about 250.

Finding and Associations

Domeikite is among the most common copper arsenides (although the mineral is generally considered rare). It often contains β-domeikite inclusions as solid solution decomposition products.

Typical associations: algodonite, nugget copper, chalcosine, bornite, galena, silver, polybasite, chalcopyrite; β-domeikite is also characterized by thin microgrowths and associations with cutinaite, koutekite, löllingite, and dyscrasite.

X-ray (basic d-lines)

α-domeikitis: 3.39 (4); 3.035 (8); 2.565 (6); 2.150 (8); 2.046 (10); 1.959 (8); 1.882 (8); 1.557 (4); 1.517 (4); 1.357 (4); 1.306 (6); 1.219 (10); 1.181 (8); 1.164 (8).

β-domeikitis: 3.64 (6); 2.345 (8); 2.214 (8); 2.050 (10); 2.002 (10); 1.959 (6); 1.432 (6); 1.244 (6); 1.184 (8); 1.174 (10).